A conventional loudspeaker utilizes a round-shaped electromagnetic transducer to drive a cone-type membrane to radiate sound. In general, an additional enclosure is necessary to facilitate sound radiation, which makes the loudspeaker cumbersome, weighty and having dead corner for sound radiation, etc. Recently, flat display and mobile communication devices such as notebook, cellular phone and personal digital assistant (PDA), are rapidly developed toward miniaturization. The integration of transparent panel-form loudspeakers with the flat display and mobile communication devices can greatly enhance the performance of such devices. Therefore, such conventional loudspeaker is gradually replaced by a panel-form loudspeaker.
FIGS. 1(a) and 1(b) are respectively top view and cross-sectional view of a traditional panel-form loudspeaker. Such panel-form loudspeaker comprises an electromagnetic transducer 10, a radiating panel 20, a frame 30, and a suspending unit 50. The transducer 10 has a resilience support 12 therein. The frame 30 is employed for supporting the transducer 10 and the radiating panel 20. The suspending unit 50 is composed of soft material to suspend the radiating panel 20 onto the frame 30.
The typical transducer for exciting a radiating panel to generate flexural vibration includes two types. FIGS. 2(a) and 2(b) illustrate cross-sectional views of two typical transducers. Each transducer comprises a cylindrical voice coil assembly 170 and a magnet assembly having at least a permanent magnet 182, at least a top plate 181 and a permeance unit 183. The voice coil assembly 170 has a moving coil 172 supported by the resilience support 12 and immersed in a magnetic field at a gap between the top plate 181, the permanent magnet 182 and the permeance unit 183. When electric current flow through the moving coil 172, the cylindrical voice coil assembly 170 will be forced to move back and forth vertically, thereby driving the radiating panel to radiate sound. In general, the resilience support 12 also works as a damper to suppress undesirable vibrations of the radiating panel 20. The transducer 10 is usually arranged at the center of the radiating panel 20 and the rigidity of the radiating panel is increased by the resilience support 12, which leads to a relatively higher initial response frequency, and considerable fluctuations of the sound pressure spectrum over the audible frequency range by exciting the radiating panel 20. In addition, when input power is augmented, a more apparent non-linear relation exists between the pressure response and the power. In order to obtain a more uniform distribution of sound pressure spectrum over the audible frequency range, U.S. Pat. No. 4,426,556 disclosed a method to excite a rectangular radiating panel by using two transducers. In such way, a more uniform distribution of sound pressure spectrum is provided. However, since locations of these two transducers are close to the short edge of the radiating panel, the radiating efficiency is reduced due to a diminished vibration.
On the other hand, the radiating panel for the traditional panel-form loudspeaker was made of metal, paper, polymer or non-woven cloth. Such materials are not suitable for producing radiating panels because they have weighty, low stiff and insufficient damping properties.